What is the meaning of the „elastic reserve” of a BFL-Mastix band ? 

The elastic reserve is the gravelfree section of the bands type R4 ½, R4 ½ D, R4S, RGD. These gravelfree surfaces have only low adhesion on the surrounding concrete and therefore allow a free deformation of the band even under important movements of the construction element (shrinkage, settling, shearing, earthquake, payloads or temperature changing). The deformation of the gravelfree band section avoids a rupture between band and concrete as well as between concrete and the gravel on the band. Thanks to their elastic reserve, BFL-Mastix bands can absorb movements that usually occur in concrete structures and can therefore assure the waterproofness of the joints. It should be stressed on the fact that BFL-Mastix bands, because of their plasto-elastic properties, absorb these movements even at very low temperatures (-20 °C/-4 °F).